To visualize the spherical (360º) content on 2D planar displays, a
selected part of the sphere must be projected into a 2D plane. The
resulting 2D area is called viewport and is created according some
fixed and limited field of view, usually much less than 360º. To
generate the viewport, two geometric projections are usually
employed: rectilinear and stereographic. For both cases, this process
leads to geometric distortions in the viewport image which, depending
on the content, may be visually unpleasant to users. Rectilinear
projection stretches the image borders while the stereographic
projection bends straight lines. In this paper, the viewport rendering
problem is tackled with the formulation of a generalized perspective
projection (which includes many other popular projection, besides
perspective and stereographic) and a perceptual optimization of its
key parameter: the projection center. The optimization process is
performed according to the viewport characteristics, namely with
perceptually important features that characterize the amount of line
distortion and stretching of salient regions. The aim is to find an
optimal, content dependent, projection center that minimizes the
content distortions and thus provides better quality of experience
(QoE) in the visualization of spherical images. Experimental results
on several 360° images show that the proposed optimized projection
achieves better results than the benchmark projections both
quantitatively and qualitatively.